Graphite is the most stable form of carbon under normal conditions, but at pressures of approximately 600,000 atmospheres and temperatures exceeding 1500.degree. C., diamond is the thermodynamically stable phase. General Electric succeeded in growing synthetic diamond in the metastable region during the early 1950's by dissolving carbon in a molten transition metal at pressures in the range of about 40,000 to about 60,000 atmospheres and temperatures in the range of about 1200.degree. to about 1600.degree. C., i.e., high pressure, high temperature (HPHT) conditions, see H. T. Hall et at., U.S. Pat. No. 2,947,610. Diamond crystals nucleate and grow from molten carbon and metal solution, typically a nickel or iron solution. While most of the diamond crystals produced by HPHT conditions are under one millimeter (mm) for diamond grit applications, several corporations have been able to produce diamond crystals almost up to one cm. Due to the enormous difficulties with scaling up the pressure in large volumes at high temperature and difficulties with preserving the integrity of seals, only very moderate improvements in the size of diamond single crystals by the HPHT technique beyond one cm can be expected in the future.
The ideal solution for the needs of the electronic industry would be hetero-epitaxial diamond on inexpensive substrates. Diamond via hetero-epitaxial chemical vapor deposition (CVD) has been reported on mm size cubic-boron nitride (C-BN) single crystal made by HPHT; see M. Yoshikawa et al., Appl. Phys. Lett., Vol. 57, page 428 (1990). C-BN shows the most promise as a heteroepitaxial substrate for diamond due to similar structure, close lattice match and high surface energy. Unfortunately, only about 1 mm size crystal of cubic BN is available from HPHT techniques as of this date and the preparation of large single crystal cubic BN substrates from gas phase is very difficult and has not as yet been accomplished.
An interesting technique has been disclosed which utilizes an array of 100 .mu.m (0.1 mm) octahedron faceted HPHT diamond crystals fitted by spinning from a slurry into corresponding pyramidal etch pits onto a silicon wafer; see N. V. Geis et al., Proceedings of the Second International Symposium on Diamond Materials, 179th Meeting of the Electrochemical Society in Washington, D.C., page 605, May 5, 1991. In this technique, commercially available (111)-faceted diamond seeds having diameters of 75 to 100 .mu.m are deposited on (100)-oriented Si substrates which had been patterned and etched using standard photolithographic methods to form 90 .mu.m square etch pits on 100 .mu.m centers faceted on (111)-planes. Homoepitaxial diamond is grown on the diamond seeds to form a continuous diamond film composed of a plurality of approximately oriented small crystals. Self supporting continuous diamond films were obtained after etching away the silicon substrate. The diamond film contains low angle grain boundaries because diamond seeds are always misoriented by a few degrees. Textured growth on silicon or beta silicon carbide has much smaller diamond grain size on the order of a few microns across the single crystal grain, but the misorientation of individual grains is on the same order of magnitude in both cases.
The disadvantages of the foregoing technique are that default holes occur in the resulting diamond film as a result of missing seed crystals in some of the etch pits and that a slight misorientation occurs among the individual single crystal grains. Therefore, this prior art technique does not produce a crystallographically perfect single crystal.
The method of the present copending parent application, U.S. Ser. No. 07/895,482 is directed to CVD diamond growth on a plurality of oriented single crystal diamond seed wafers which are patterned with a plurality of seed holes formed in such a way that newly grown single crystal diamond layer can be separated from a reusable diamond substrate. As a consequence of the orientation of two or more precisely oriented single crystal diamond patterned structures, a large diamond single crystal seed plate is generated by epitaxial fusion.
R. A. Rudder et al., has succeeded in depositing diamonds onto a photolithographically defined large electronic device area and has observed isotropic overgrowth advancing vertically and horizontally by about the same rate; see R. A. Rudder, J. B. Posthill, G. C. Hudson, D. Malta, R. E. Thomas, R. J. Markunas, T. P. Humphreys, R. J. Nemanich, Proceedings of the Second International Conference New Diamond Science and Technology, Washington, D.C., page 425, Sep. 23-27, 1990. The Rudder et al. method includes the steps of depositing polycrystalline silicon onto a single diamond substrate; depositing a masking layer onto the Si layer; photolithographically opening holes to allow for the homoepitaxial diamond to nucleate; and depositing diamond onto the resulting substrate by CVD to form overgrowth extending over the Si pattern.
The preceding reference does not teach separating the resultant deposited diamond layer from the substrate by any means such as mechanical, physical or cutting means. The reference also fails to teach growing a continuous monocrystalline (i.e., a single crystal) layer of material onto a substrate and then separating that material from the substrate. The quality of homoepitaxially grown diamond above the holes in the masking layer on the device was claimed to be superior to the quality of underlying diamond substrate; see J. B. Posthill, R. A. Rudder, G. C. Hudson, D. P. Malta, G. C. Fountain, R. E. Thomas, R. J. Markunas, T. P. Humphreys, R. J. Nemanich, Proceedings of the Second International Symposium on Diamond Materials, Proceedings Vol. 91-8 of the Electrochemical Society, May 5-10, 1991, page 274, Washington, D.C.
The superior quality of a laterally propagated epitaxial layer is believed to be due to the so called "necking effect", which is frequently used in Bridgman or Czochralski crystal growth. Necking down the growing crystal limits the propagation of dislocation from the seed crystal only to the straight direction of growth, but not in lateral directions. In the references mentioned above, R. A. Rudder and J. B. Posthill have demonstrated that the same effect of lateral overgrowth which is being used successfully in silicon and gallium arsenide microelectronics for the fabrication of three dimensional integrated circuits can be used to fabricate three dimensional integrated circuits in diamond microelectronics technology. The reference does not teach using this technology to grow large monocrystalline diamond plates.
Anthony et al., U.S. Pat. No. 5,264,071, teach a chemical vapor deposition (CVD) method for making a monolithic polycrystalline (i.e., multiple crystals) diamond sheet in which a diamond is deposited onto a non-diamond substrate which is smooth and free of surface irregularities to reduce physical bonding between such irregularities to the deposited polycrystalline CVD diamond. The polycrystalline diamond sheet is separated from the substrate by a cooling step or by dissolving the entire substrate after the deposition to facilitate the release of the diamond sheet from the polished, metallic substrate.
The electronic industry is using large semiconductor single crystal wafers ranging in size from 5 cm to 20 cm in diameter. Wafers smaller than 5 cm in diameter currently cannot be economically produced. Therefore, for the electronic industry desires free-standing single crystal diamond wafers which are larger than 5 cm in diameter, so that the electronic industry can take advantage of the outstanding electronic properties of diamond. In order to take full advantage of diamond's outstanding electronic properties, diamond large free-standing single crystal wafers must be true single crystals which do not possess large angle grain boundaries and should be at least comparable in quality to the best natural diamond single crystal. Polycrystalline diamond films which deviate from the ideal orientation of single crystal by as little as 0.2 degree will display inferior electronic properties by the virtue of the presence of large angle grain boundaries. Large angle grain boundaries cause impurities to segregate on the interface between the grains and display an undesirable concentration of structural defects and electrical defects. These textured quasi-epitaxial diamond films or crystalline diamond structures demonstrate inferior electrical properties.